1. Field of the Invention
This invention relates generally to door sills, which are commonly referred to as thresholds, and specifically to a structural component of the sill for mounting door jambs at opposite ends of the sill.
2. Description of the Related Art
Door sills have been used for many years to seal the bottoms of swinging residential doors when they are in the closed position. Sills originally consisted of multiple parts, including separate sill and threshold portions. This distinction in parts arose from the different functions the parts performed when door frames were constructed on site.
In the past when a door frame was being constructed, the carpenter fastened the door jambs in place on opposite sides of an opening roughly framed in a wall. Subsequently, an upper cross piece was fastened between the upper ends of the jambs, and then the sill was placed against the floor between the lower ends of the jambs. Once the sill was in place, the threshold was fastened onto the sill, and then the door was hung by fastening it to one of the jambs.
Improvements were made to sills which were to be fastened in place when the doorframe was constructed on site. Miller, in U.S. Pat. No. 2,696,028, discloses a door sill which extends out of an exterior wall. Erkkila, in U.S. Pat. No. 2,875,481, shows a similar sill which has similar advantages. Stevenson, in U.S. Pat. No. 794,953 and Headrick et al., in U.S. Pat. No. 5,611,173, show improvements in door sills.
Since the late 1960's and early 1970's, virtually all door frames constructed in the United States have been pre-hung, i.e. premanufactured prior to delivery to a construction site. In manufacturing facilities, a pair of door jambs are mounted to a sill and an upper door frame member to form a door frame. Typically, the sill has an extruded aluminum sill frame with a wooden plank attached in a cavity in the underside thereof. During manufacture, the lower, inner sides of the door jambs are seated against the ends of the sill and the lower ends are aligned flush with the lowermost extreme face of the sill. Then nails, staples or screws are fastened through the outside jamb edge into the sill's wooden plank. A cross member is then fastened in place between the upper ends of the jambs and the door is mounted in the frame. The entire unit is then transported to the construction site where it is fastened in a rough door opening. The Erkkila and Miller sills could not be used in the pre-hung door process without significant alterations to the sills and the frame members attached to the sills.
One disadvantage with a pre-hung door is the tendency of the frame to become misaligned with the door due to weaknesses in the joints at the corners of the frame. Because the door has outside corners which are at 90 degree angles, the frame should have inside corners which are at 90 degree angles. In order for the door to open and close properly, there must be sufficient gaps between all four outer door edges and the inner edges of the adjacent frame members. When the door frame corners are not at 90 degree angles due to weaknesses in the corner joints, the gaps between the door and the frame becomes too small for door movement. This misalignment requires door removal and trimming or complete replacement of the door and frame.
A second disadvantage with pre-hung doors is the tendency of the jambs to rot due to water contact. The lower ends of the wooden jambs are aligned with the lowest extreme of the sill in order to maximize the area of contact between the door jamb and the sill end, which maximizes the joint's strength. However, this often has a negative result. The lower ends of the jambs touch the subfloor of the door opening, which permits any water that comes into contact with the lower ends of the jambs to wick up into the jambs. The grain of the wooden jamb is aligned along the jamb's length, which permits this wicking of water to occur by capillary action.
Therefore, the need exists for a door sill or frame which is strong, and which prevents wicking of water from the subfloor to the jambs.